SINCLAIR— THE " TURTLE^OREODON LAYER." 465 



toward dryer conditions, bringing about the extinction of Titano- 

 therimn. 



With the return of fluviatile deposition, a slower accumulation 

 of silt by sheet-flood action over a level plain may be indicated by 

 the predominance of approximate horizontality in the stratification 

 and uniformity in the thickness of the color-banding. The slight 

 southeasterly dip which the beds show (Plate VII, Fig. 2), inclining 

 them at a greater angle than the present high-prairie top, is probably 

 initial dip, possibly intensified by subsequent warping. Seasons or 

 cycles of less abundant precipitation, during which ground water 

 was drawn surfaceward by capillarity, are indicated by the zones of 

 coliche nodules. Perhaps the pink color of the basal clays of the 

 Creodon beds, the " turtle-oreodon layer " or " red layer " we have 

 been discussing, has a climatic significance, but considering how 

 little we know about the chemical changes involved in the iron com- 

 pounds responsible for the color or the conditions under which they 

 take place, it would be unsafe to conclude that it indicated aridity, 

 for our modern western-desert sediments are gray and not red. 

 Certain pink clay zones farther up in the Oreodon beds, shown as 

 darker bands in the distance in Fig. 2, perhaps owe their color to 

 climate, but what kind of climate must remain, for the present, 

 doubtful. The testimony of mouse-nibbled bones, carnivore excre- 

 ments and carcasses in death pose fits in with the climatic rhythm 

 here suggested. 



The zone of green channel-sandstone lenses weathering into 

 spherical concretions, which occurs above the basal pinkish-gray 

 clays of the Oreodon beds in the Big Badlands, may represent a 

 swing-back toward the pluvial cycle. Minor rhythms are, I think, 

 iridicated by certain of the larger channel fillings, such as the Meta- 

 mynodon channels, filled with coarse sediments and afifording re- 

 mains of an aquatic rhinoceros. These may have been deposited 

 during cycles of greater precipitation about the headwaters of the 

 streams, which sent them, charged with coarse sediments, far out 

 over the clay-covered plain to the east. 



The recurrence of Metamynodon in channels at dififerent levels, 

 noted by Wortman,^ fits in with the interpretation just suggested, 

 ^Buli Am. Mus., Vol. V., Article IX., p. loi, 1893. 



